Asthma and other respiratory diseases have long been treated by the inhalation of appropriate medicaments. For many years the two most widely used and convenient choices of treatment have been the inhalation of medicament from a drug solution or suspension in a pressurized metered dose inhaler (pMDI), or inhalation of powdered drug, generally admixed with an excipient, from a dry powder inhaler (DPI). Following strong concern about the link between depletion of the earth's ozone layer and chlorofluorocarbon (CFC) emissions, the use of these materials in pressurized inhalers is being phased out and interest in DPI systems has been stimulated.
Current regulatory guidance in some parts of the world recommends that a medicinal inhaler include a dose-counting device so that a patient may be aware when a device is nearing the end of its recommended number of actuations. It is desired that a dose counter be as nearly 100 percent reliable as possible and, in particular, that a device avoid undercounting. That is, the device should not deliver a dose without advancement of the counter, since this could lead a patient to believe there is more medication left in the device than actually remains.
Dose counters for dry powder inhalers are well known in the state of the art, including those described in, for example, U.S. Pat. No. 5,582,162 (Petersson), U.S. Pat. No. 5,590,645 (Davies et al.), U.S. Pat. No. 7,107,988 (Pinon et al.), U.S. Pat. No. 7,322,352 (Minshull et al.), and WO 2005/079727 (Augustyn et al.). In practice most DPIs use either bulk powder reservoirs or individual pre-measured doses sealed within individual containers in the form of capsules or blisters, such as blister packs and blister strips. In a typical device with capsules or blisters it is generally sufficient to simply count the capsule or blisters one by one as the device is actuated and they are used. In a typical reservoir device there is generally a metering step, such as rotation of a dose cup from a position inside the reservoir to a position within an airflow chamber, and dose counting can be done by simply counting the number of rotations of the dose cup.